The present invention involves a method for the reduction of sulfur oxides in a coal gasification process.
In view of recent increases in the price of crude oil, researchers have been searching for alternative sources of energy and hydrocarbons. Much research has focused on recovering the hydrocarbons from hydrocarbon-containing solids such as shale, tar sand or coal by heating or pyrolysis to boil off or liquefy the hydrocarbons trapped in the solid or by reacting the solid with steam, for example, to convert components of solid carbonaceous material into more readily usable gaseous and liquid hydrocarbons. Other known processes involve combustion of the solid carbonaceous materials with an oxygen-containing gas to generate heat. Such processes conventionally employ a treatment zone, e.g., a reaction vessel, in which the solid is heated or reacted.
In a typical coal gasification process, coal is contacted with steam and an oxygen-containing gas to produce a gaseous product. The sulfur in the coal is emitted as hydrogen sulfide.
When air is used as the oxygen-containing gas, the gaseous product contains high levels of nitrogen, which reduces the BTU content of the gaseous product. Some processes have used pure oxygen instead of air, in order to avoid having nitrogen in the gaseous product. This does eliminate the nitrogen from the product but it requires a source of pure oxygen, some oxygen plants are almost as large as the coal gasification plant they are supplying. Thus, one was faced with the alternatives of either producing a gaseous product diluted with nitrogen or finding a source of pure oxygen for their process.
Another solution to the nitrogen dilution problem is disclosed in U.S. Pat. No. 4,157,245. In one embodiment of the invention disclosed in that patent, a solid heat-transfer material, such as sand, is introduced into an upper portion of a reaction vessel and coal is introduced into a lower portion of the vessel. The physical characteristics of the heat-transfer material and the coal differ such that a superficial velocity of a fluid flowing upwardly through the vessel is greater than the minimum fluidizing velocity of the heat-transfer material and the terminal velocity of the coal, but is less than the terminal velocity of the heat-transfer material. A substantially countercurrent vertical flow of the two solids is maintained in the vessel without substantial top-to-bottom backmixing by passing steam upwardly through the vessel at a rate sufficient to fluidize the heat-transfer material and entrain the coal whereby the heat-transfer material substantially flows downwardly in a fluidized state through the vessel and the coal substantially flows upwardly in an entrained state through the vessel. The steam reacts with the coal to form a hot char and a gaseous product, with some of the sulfur in the coal being emitted as hydrogen sulfide. The heat-transfer material acts as a source of heat for the reaction between the steam and the coal. Cooled heat-transfer material is removed from a lower end of the vessel and the hot char, the gaseous product and the hydrogen sulfide are removed from an upper end of the vessel. The gaseous product is then separated from the hot char and the hydrogen sulfide by regular separation techniques.
In one method, the heat-transfer material can be heated by introducing it into an upper portion of a combustion zone, introducing the hot char into a lower portion of the zone, and contacting the heat-transfer material with the hot char while maintaining substantially countercurrent plug flow of the two solids by passing air upwardly through the combustion zone at a rate sufficient to fluidize the heat-transfer material and entrain the char. The heat-transfer material substantially flows downwardly through the combustion zone in a fluidized state and is heated while the char substantially flows upwardly through the combustion zone in an entrained state and is combusted, with some of the sulfur in the char being released as sulfur oxides.
The process in U.S. Pat. No. 4,157,245 is based in part on the discovery that in the typical coal gasification process, there are two separate reactions occurring in the same vessel: (1) an endothermic reaction between the coal and steam which produces the gaseous product, and (2) an exothermic reaction between the coal and the oxygen-containing gas which produces the heat necessary for the first reaction. The process of U.S. Pat. No. 4,157,245 separates these two reactions in two separate vessels and transfers the heat generated by the second reaction to the site of the first reaction via a heat-transfer material.
A major advantage of this process is that air can be used as the oxidizing gas without causing the resulting gaseous product to be diluted with nitrogen. A disadvantage of this process is that sulfur-containing gases are coming out of two vessels.
In the typical process wherein coal is contacted with oxygen and steam in the same vessel, sulfur which is contained in the coal is emitted as hydrogen sulfide, which can be cleaned from the gaseous product stream by known technology. In the process disclosed by U.S. Pat. No. 4,157,245, hydrogen sulfide is produced in the first reaction vessel, where the coal is contacted with the steam, and sulfur oxides are formed in the combustion zone, where the char is combusted with air or oxygen, and is released with the flue gas. Since the sulfur oxides must be removed before the flue gas can be released, it would be advantageous to transfer the sulfur oxides in the combustion zone to the reaction zone and release it as hydrogen sulfide so that it could be cleaned up by the same process equipment that removes hydrogen sulfide from the gaseous product stream.